Regression analysis in SPSS
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In a study with three treatment groups, A1, A2 and A3, and a control group, C, dummy vectors were constructed as follows: subjects in A1 were identified in D1, those in A2 were identified in D2, and those in A3 were identified in D3. A multiple regression analysis was done in which the dependent variable was regressed on the three coded vectors. The following regression equation was obtained:
y = 8 + 6D1 + 5D2 -2D3
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This solution gives a step by step method for computing a regression model in SPSS.
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Part II: Short Questions.
1. In a study with three treatment groups, A1, A2 and A3, and a control group, C, dummy vectors were constructed as follows: subjects in A1 were identified in D1, those in A2 were identified in D2, and those in A3 were identified in D3. A multiple regression analysis was done in which the dependent variable was regressed on the three coded vectors. The following regression equation was obtained:
y = 8 + 6D1 + 5D2 -2D3
(a) Which group is used as the reference group?
Reference group is the Control Group
(b) What are the means of the four groups on the dependent variable?
Group Mean
Control group 8
A1 8+6=14
A2 8+5=13
A3 8-2=6
2. The following regression equation was obtained from an analysis with effect coding for four groups with equal sample size:
y = 102.5+2.5E1-2.5E2-4.5E3
(a) What is the grand mean of the four groups?
Grand Mean =constant =102.5
(b) What are the means of the four groups, assuming that the fourth group was assigned -1's?
The coefficients of each of the effect variables is equal to the difference between the mean of the group coded 1 and the grand mean.
Group Mean
A1 102.5+2.5=105
A2 102.5-2.5=100
A3 102.5-4.5=98
(c) What is the effect of each treatment?
The effect of each treatment ...
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